A Guided Tour of Ethereal

Learn exactly what's in all those packets flying by on your network with this essential development and administration tool.

I recently started using a network tool called Ethereal. For those
familiar with tcpdump, think of Ethereal as
a GUI form of tcpdump that shows you the whole packet and can break
down the packet to show individual fields. For those who haven't
used tcpdump or similar packet sniffers, it might be best to show
the capabilities of Ethereal through a few examples.

When you start Ethereal, it looks like the graphic shown in Figure
1.
Typically, you want to capture some data from
the network attached to your workstation; do this by selecting
Capture→Start..., which brings up the dialog shown in Figure 2. When
you've captured the data you need, stop the capture and
examine it. Figure 3 shows a capture of some IPv6 traffic, where I've
selected an ICMPv6 packet (in the top frame) and expanded the
IPv6 and ICMPv6 contents to select the IPv6 source address (in the
middle frame). Ethereal automatically highlights the raw
bytes corresponding to the selected field—in this case, source
address—within the packet in the bottom frame. This type of
functionality makes Ethereal useful for understanding various
network protocols, and I definitely recommend its use as a teaching or
self-education aid in conjunction with networking RFCs. Ethereal also is
useful for educating users and management about the dangers of
using protocols that send data in clear text, as shown for File
Transfer Protocol in Figure 4.

Figure 1. The Ethereal Main Window

Figure 2. Capture Dialog

Figure 3. ICMPv6 Capture and Dissection

Figure 4. FTP Capture and Dissection, Showing Password

Ethereal also is useful for investigating proprietary protocols or
other networking protocols that are not well documented. Figure 5
shows a somewhat contrived example—rsync. This protocol is in
widespread use because of its ability to save significant bandwidth but is essentially defined by the source code to the application. I
used Ethereal to capture a number of rsync transactions and figured
out how the protocol works—at least enough to write an rsync protocol
dissector for Ethereal. I understand the Samba team uses
Ethereal and a number of other tools to develop clients and servers
that interoperate with the Microsoft CIFS implementations, because the
Microsoft documentation for these protocols is incomplete or
incorrect.

Figure 5. Ethereal Capturing rsync (Now Supported)

I also have used Ethereal as a part of network application testing (on
zcip and Service Location Protocol) to assess correctness and
response times. Ethereal time-tags each transaction, so you easily can
see the relationship between packets.

How Ethereal Works

Ethereal works by capturing packets through a reasonably portable
library called libpcap, which on Linux accesses the packets on the
network through using a kernel mechanism called packet socket.
It is possible to disable this option under Linux, although probably all
vendor kernels have it enabled, and it is enabled in the default
kernel configuration for most architectures on Linux
kernels. Other operating systems have different interfaces, but
libpcap abstracts this away and provides a common API.

Having received a copy of the network packets, Ethereal builds an
internal linked list and saves the packets to a file. It then
determines what protocol the packet is carrying based on the port
numbers, type fields in the supporting protocols or a heuristic that
guesses the protocol based on the contents of the field. It is worth
noting that this approach essentially is informed guesswork and is by
no means infallible. For example, traffic to port 53 probably is DNS,
but there is no reason why a network administrator could not choose to
run another service on that port. In addition, Ethereal supports
an option to interpret a particular packet as a different protocol,
using Tools→Decode As.

Based on the guessed protocol, Ethereal decodes (dissects, in
Ethereal nomenclature) the packet. Each protocol supported by Ethereal
is handled through a bit of code known as a dissector. At the time
of this writing, 333 dissectors are built in to Ethereal, some of
which handle more than one protocol. Protocols also can be provided as
plugins, which are loaded dynamically. Depending on the protocol
and the level of sophistication provided by the dissector code, the
packets can be broken down for analysis of individual bits or they can be
presented at a very high level. Both options are depicted in Figure 6,
where the TCP dissector shows the individual bits set in the flags,
but the IMAP dissector breaks out only two fields. It is worth noting
that IMAP is a text-based protocol, so a simple ASCII dump of the
packet contents is an appropriate way to show them.

Regarding the ability to capture packets remotely:
While it's true that Ethereal cannot do this dynamically, i.e.; with an agent on the remote end, Ethereal can read packet captures from command line tools such as tcpdump and snoop.

I use both of these tools to capture packets from Firewalls, Routers, servers, etc. I also use a beat-up Pentium-90 laptop as a network monitor that I can leave at a customer site. Once the data is collected I can analyse it with Ethereal. Ethereal will also read packet captures from commercial tools such as NAI's Sniffer tools.

Ethereal is a tool that just keeps getting a little better each year. I've used it to solve a variety of problems, but I've also used it to teach networking protocols. It's the best tool I know of to show students exactly how protocols are encapsulated in each other and to demonstrate exactly how data gets across the network.

On a slightly different note, it's interesting that I'm posting this comment on January 10th 2004, but the article claims to have been posted on Feburary 1st, 2004.

I think the date reflects the publishing date for the magazine, not for the article.

I agree with the remote capture comments, and some work on remote capture has been done, but when you are working with the Ethereal GUI, it would sometimes be nice to do "now show me what that remote machine is seeing, in real time". That needs more work.

Trending Topics

Webinar: 8 Signs You’re Beyond Cron

Scheduling Crontabs With an Enterprise Scheduler
11am CDT, April 29th

Join Linux Journal and Pat Cameron, Director of Automation Technology at HelpSystems, as they discuss the eight primary advantages of moving beyond cron job scheduling. In this webinar, you’ll learn about integrating cron with an enterprise scheduler.